Maintaining the composition of transferred data during handover

Abstract

In a radio data transmission system which transmits data in cell form, the risk of cell loss or the risk of out-of-ordering during handover can be diminished by synchronizing the cell transmission between the new (50) and the old (40) base stations. The old base station (40) delivers to the new base station (50) and/or the switch (51), information on the last cell that was transmitted successfully and in the right order through the old base station, whereby the new base station continues the transmission by starting from the first failed cell. Cells need not be identified one by one if group numbering is applied in which the first or the last cell of a given group (31, 32, 33, 34) or a given counter cell is numbered, and the rest can be indicated, where necessary, by counting the distance from the numbered cell.

We claim: 
     
       1. A method for implementing a handover in a data transmission system that comprises a switch (51) and a first (40) and a second (50) base station that are affiliated with the switch, and a mobile unit (41) that is affiliated wirelessly with the base stations,   and wherein the operation of the data transmission system comprises the transmission of data as cells of a given size, and the handover comprises the transmission of said cells of data first between the mobile unit (41) and the first base station (40) and, after that, between the mobile unit (41) and the second base station (50), said method comprising the steps of: transmitting a flow of said cells between the first base station (40) and the mobile unit (41);   detecting the first cell that is not successfully transmitted between the first base station and the mobile unit and subsequent cells that are not successfully transmitted, to determine which cells have been successfully transmitted between the first base station and the mobile unit; and   providing information from the first base station (40) to the switch (51) and/or the second base station (50) on which cells have been successfully transmitted between the first base station and the mobile unit.     
     
     
       2. A method according to claim 1, wherein the transmission of cells between the second base station and the mobile unit starts g from the first cell that is not successfully transmitted between the first base station and the mobile unit. 
     
     
       3. A method according to claim 1, wherein in the first (40) and the second (50) base stations, downlink cells are stored in respective buffers (42, 52) before transmitting them to the mobile unit (41). 
     
     
       4. A method according to claim 3, wherein when transmitting downlink cells from the first base station (40) to the mobile unit, the mobile unit (41) acknowledges the cells received successfully, whereby the downlink cells that are acknowledged as having been received successfully and in the right order are cleared from the buffer (42) of the first base station. 
     
     
       5. A method according to claim 4, wherein during the handover, the downlink cells, which have not been cleared from the buffer (42) of the first base station, are transmitted to the second base station (50) to be sent to the mobile unit (41). 
     
     
       6. A method according to claim 1, wherein the switch (51) adds a first message (D) to the transmission flow consisting of downlink data cells to be transferred, the message (D) indicating the termination of the cells to be transmitted through the first base station (40), and the start of the cells to be transmitted through the second base station (50), whereby the location of the first message (D) is fixed with respect to the downlink cells of said transmission flow. 
     
     
       7. A method according to claim 1, wherein the switch (51) adds counter messages (C1, C2) to the transmission flow comprised of downlink data cells to be transferred, the location of the counter messages being fixed with respect to said downlink cells of the transmission flow. 
     
     
       8. A method according to claim 7, wherein the transmission flow comprises numbering cells having serial numbers, and wherein said counter messages (C1, C2) comprise a serial number that has been added to a given group of numbering cells and that deviates from the immediately preceding serial numbers, and there is, respectively, a predetermined number of cells between said numbering cells, belonging to the group of cells (31, 32, 33, 34) indicated by said serial number. 
     
     
       9. A method according to claim 7, wherein the switch (51) transmits the same cells both to the first (40) and to the second (50) base stations, whereby both base stations store said cells in respective buffers (42, 52), and after the first base station (40) has successfully sent all the cells that preceded a given counter message (C1) to the mobile unit (41), the first base station transmits a second message that reports this to the second base station (50), and as a response to said second message, the second base station clears the corresponding cells from its buffer (52). 
     
     
       10. A method according to claim 1, wherein uplink cells in the mobile unit (41) are stored in a buffer before transmission. 
     
     
       11. A method according to claim 10, wherein when transmitting uplink cells from the mobile unit (41) to the first base station (40), the first base station acknowledges the successfully-received cells, whereby the uplink cells that are acknowledged to have been received successfully and in the right order are cleared from said buffer of the mobile unit. 
     
     
       12. A method according to claim 10, wherein the first base station (40) transmits, during the handover, a termination message (U) to the switch (51), the termination message indicating the termination of the successfully transmitted cells through the first base station (40). 
     
     
       13. A method according to claim 1, wherein the mobile unit (41) adds counter messages to the transmission flow comprised of uplink data cells to be transferred, the location of the counter messages being fixed with respect to the uplink data cells of said transmission flow. 
     
     
       14. A method according to claim 13, wherein the transmission flow comprises numbering cells having serial numbers, and wherein said counter messages comprise a serial number that has been added to a given group of numbering cells and that deviates from the immediately preceding serial numbers, and that there is, respectively, a predetermined number of cells between said numbering cells, belonging to the group of cells indicated by said serial number. 
     
     
       15. A method according to claim 13, wherein the mobile unit (41) sends the same uplink cells to both the first (40) and to the second (50) base stations, whereby both base stations store the uplink cells in respective buffers, and after the first base station (40) has successfully received all the cells that preceded a given counter message from the mobile unit (41), the first base station transmits another message reporting this to the second base station (50) and, as a response to said another message, the second base station (50) clears the corresponding cells from its buffer. 
     
     
       16. A base station (40, 50), of a radio data transmission system having a plurality of base stations, for transmitting data in cell form and in a right order to a switch (51) connected to the base station by means of a wired connection and to a mobile unit (41) which is in radio link with the base station, comprising: means for detecting the first data cell that is not successfully transmitted between said base station and the mobile unit and subsequent data cells that are not successfully transmitted, to determine which cells have been successfully transmitted between said base station and the mobile unit;   means for delivering to other base stations of said plurality of base stations the data on cells that are transmitted successfully and in the right order between said base station and the mobile unit, and   means for receiving the corresponding data from said other base stations.   
     
     
       17. A base station according to claim 16, further comprising a buffer entity (42, 52) for temporarily storing the data in cell form, and a means for selectively clearing cells from said buffer entity. 
     
     
       18. In a radio transmission system comprising a switch (51) for controlling data in cell form, and at least two base stations (40, 50) that are affiliated with the switch, for transmitting the data in cell form between the switch (51) and a mobile unit (41), the mobile unit communicating wirelessly with the base stations, the improvement comprising: means for detecting the transfer of data in cell form between one of said base stations and said mobile unit and producing information regarding which cells have been successfully transferred; and   means, responsive to the information produced by said detecting means, for transmitting messages concerning the successfully transferred cells between the base stations.